This invention relates to sampled data filters and more particularly to apparatus for reducing pre- and post-signal overshoot in the filter output response attendant sharp signal transitions applied to such filters.
Typically sampled data filters operate by successively producing weighted sums of successive groups of input samples. This is accomplished in an output weighted filter, for example, with a delay element having a plurality of output taps at which successive signal samples delayed by successively greater delay periods can be accessed. The samples from the various taps are weighted and then added together to provide the filtered replica of the input sample. Nominally, the filtered replica of an input sample x(n) presently within the confines of the filter is the weighted sum of input samples x(n+i) occurring in time before the sample x(n) and input samples x(n-i) occurring in time after the sample x(n). If the particular sample x(n-j) represents a large amplitude transition, it will readily be appreciated that it can significantly influence the value of the filtered output sample y(n). In other words, it is a characteristic of the output signal of the sampled data filter to undergo output signal changes responsive to input changes in advance of the filter group delay. Depending on the design of the particular filter, the artifacts produced by the anticipatory nature of the sampled data filter may be particularly objectionable. For example, if the filter is designed with symmetrical tap weights about the filter center point and includes both positive and negative tap weights, a positive going signal transition x(n-j) may produce a negative going filter output transition (undershoot) in advance of the filtered replica of the transition and an overshoot subsequent to the filtered replica of the transition. Conversely a negative input transition may create a pre-overshoot and a post-undershoot. The magnitude and duration of the under and overshoots can be readily calculated for any given input transition knowing the values of the filter tap weights. It is an object of the present invention to reduce output signal artifacts due to particular input signal transitions in sampled data filters.